As with many industries, there is a finite pool of owner drivers with high quality vehicles working the airport limo services. What tends to happen, is the incumbent provider of a service to a particular airline or customer has to re-tender for the contract every 2-3 years. The airlines choose mainly based on lowest tender price. As soon as the contract is switched, many of the drivers jump ship, so you end up getting pretty much the same service anyhow.

This is also largely true with mishandled baggage delivery, airline security, contract bussing etc.